There have been great strides made in the study of laser in recent years. In particular, solid lasers and semiconductor lasers that can emit light of wavelengths ranging from the near infrared region to the infrared region are available in the form of high-powered, small-sized laser devices. Such laser devices are remarkably useful as the light sources for light exposure when printing plates are made by direct transfer of digital data from computers or the like to image forming materials.
Conventionally, a light-sensitive image forming material of a positive-working mode designed for direct plate making, which is used with infrared laser comprises, for example, a novolak resin as an alkali-soluble resin. For example, Japanese Patent Application Publication (hereunder referred to as “JP KOKAI”) No. Hei 7-285275 discloses a light-sensitive image forming material of a positive-working mode. The above-mentioned image forming material comprises an alkali-soluble resin having a phenolic hydroxyl group, such as a novolak resin, to which are added light-sensitive compounds of positive-working mode, i.e., a material capable of generating heat after absorbing light, a variety of onium salts, and quinone diazide compounds. At a non-light exposed portion (which will serve as an image portion) in the image forming material, the above-mentioned light-sensitive compounds of positive-working mode serve as a dissolution blocking agent, in other words, work to substantially decrease the solubility of the alkali-soluble resin. At a light exposed portion (which will become a non-image portion), on the other hand, the light-sensitive compounds do not exhibit any dissolution blocking performance by the generation of heat, so that the image forming material is allowed to dissolve in a developing solution, to complete the removal of the image forming material at the non-image portion. Thus, image formation is achieved.
Further, there is proposed a light-sensitive image forming material of positive-working mode comprising a substance capable of absorbing light to generate heat and a resin of which the solubility in alkaline aqueous solutions is changeable depending upon the application of heat as disclosed, for example, in International Publication No. WO 97/39894 and EP-A-0823327. This type of image forming material can achieve image formation in such a manner that one portion of the image forming material corresponding to an image portion exhibits a low solubility in an alkaline aqueous solution, while the other portion which will be a non-image portion shows an increased solubility in the alkaline aqueous solution by the generation of heat, thereby readily allowing the portion corresponding to the non-image portion to be removed by development.
However, in the above lithographic printing plate precursor of positive-working mode for infrared laser scanning, there is a problem that it is difficult to make highly sharp and clear images, because a difference of dissolving rate in an alkaline developing solution between a light-exposed portion and an unexposed portion is small. Furthermore, in a heat-sensitive printing plate precursor as described above, there is a problem that excessive development or defective development tends to take place depending on variation of use conditions, since a difference between a resistance of dissolution residing in an unexposed portion (image portion) to a developing solution and a solubility of a light-exposed portion (non-image portion) in a developing solution is still insufficient in various use conditions. In addition, the image-forming ability of the lithographic printing plate precursor depends on a heat generation on the surface of an image recording layer through an infrared laser exposure, and in the vicinity of a substrate, quantity of heat which is used for dissolution of the image recording layer, i.e., for an image formation is small due to a dispersion of heat, resulting in that a difference between light-exposure and unexposure becomes small and a reproducibility of highlight becomes insufficient.
In consideration of the above problems, it has been proposed to add a specific compound, i.e., a development inhibitor to a developing solution composition, said compound preventing the image area on the lithographic printing plate from being etched, as described, for example, in JP KOKAI No. Hei 11-194504. Such a development inhibitor may interact with a compound dissolving from a light-sensitive layer into a developing solution, and then alkali-solubility of said compound dissolving from the light-sensitive layer is remarkably decreased to generate insoluble matter. Such insoluble matter becomes sludge in a processing bath for development, and when such sludge adheres to a non-image area of printing plate, said condition may cause scumming during printing. On the other hand, the development inhibitor is consumed through the interaction with the compound dissolving from a light-sensitive layer into a developing solution, resulting in that the advantages of inhibiting development by the development inhibitor is not maintained. Further, EP-A-0274044 has proposed an alkaline developing solution comprising ethyleneoxide/propyleneoxide block copolymer and optionally a phosphate ester and the like, and JP KOKAI 2001-242639 has proposed a technique that a specific phosphate ester compound is added to a developing solution composition to improve an ability of image formation. Though the above compounds are used, image forming performance or inhibition of occurrence of development sludge is not sufficiently accomplished.
In the art of image forming materials, a novolak resin is conventionally used preferably, and recently as a more preferable image forming material, a light-sensitive image forming material comprising a novolak resin comprising xylenol as a constituent monomer is proposed, for example, in JP KOKAI No. 2001-350261. The use of said novolak resin comprising xylenol as a constituent monomer can lead to an excellent time-lapse stability of the prepared lithographic printing plate precursor. On the other hand, the above-mentioned novolak resin containing as a monomer component xylenol shows a lower solubility in alkaline aqueous solutions as compared with the conventional novolak resins. Therefore, when this type of novolak resin is used for the image forming material, the image forming material constituting a non-image portion once dissolved in the developing solution tends to form insoluble matter in the developing solution. Such insoluble matter is accumulated and aggregated to form development sludge in the developing solution after repetition of the development steps, which becomes a factor that will make the development step unstable. To be more specific about the disadvantages caused by the above-mentioned phenomenon, the development sludge can be deposited on the resultant plate to impair the images formed thereon, and the development sludge which settles down and precipitates in a processing bath for development makes the maintenance of the processing bath more burdensome. In the case where the PS plate is subjected to a burning treatment, the development sludge, if still remaining on the non-image portion of the PS plate, will be carbonized, thereby resulting in scumming, in other words, staining the printed matter.
Thus, in making a lithographic printing plate, there has been required a method in which the occurrence of development sludge caused by a component of image recording layer and the like is inhibited, a highly sharp and clear image is formed without impairing the formed image areas, and stable development procedure and stable image formation are accomplished for a prolonged period.